In various electrochemical devices such as polymer electrolyte fuel cells or water electrolytic devices, a solid polymer electrolyte is shaped into a membrane, to both sides of which electrodes are each bonded, and is then used in the form of a membrane electrode assembly (MEA). Also, in a polymer electrolyte fuel cell, an electrode generally has a double-layered structure with a diffusion layer and a catalyst layer. The diffusion layer serves to supply reaction gas and electrons to the catalyst layer, for which carbon fiber, carbon paper, or the like is used. Furthermore, the catalyst layer provides a reaction field for an electrode reaction, which generally comprises a complex of an electrode catalyst and a solid polymer electrolyte.
For such electrode catalyst to be used for various electrochemical devices, fine particles (e.g., Pt black) of a noble metal such as Pt, carbonaceous (e.g. carbon black) carriers onto which fine particles of a noble metal such as Pt are loaded, and noble metal thin films formed on electrolyte membrane surfaces by a method such as metal plating or sputtering have been conventionally used, for example.
However, noble metals such as Pt show high catalytic activity and stability in high catalytic activity, but are expensive and limited as resources. Hence, this is a reason such that the electrode catalyst increases the cost of various electrochemical devices. In particular, fuel cells are used in a form in which many MEAs are stacked, so as to obtain predetermined output. Thus, the amount of an electrode catalyst to be used per fuel cell is high. This inhibits the spread of such fuel cells.
Various proposals have been made to address the problem.
For example, JP Patent Publication (Kokai) No. 2005-285511 A discloses a method for producing a fuel cell, comprising:
(1) adding Pt-loaded carbon black powder (Pt/C) to a hydrogen peroxide solution;
(2) further adding an electrolyte polymer to the solution when oxygen generation ceases, agitating the solution for 30 minutes, and then allowing the solution to stand so as to precipitate Pt/C particles;
(3) decanting the supernatant solution and then collecting electrolyte polymer-coated Pt/C;
(4) preparing a paste for electrodes using the thus obtained electrolyte polymer-coated Pt/C, coating a diffusion layer with the paste for electrodes, and then bonding the coated layer to the electrolyte membrane.
JP Patent Publication (Kokai) No. 2005-285511 A describes the following: that since a hydrophilic group is introduced onto the C surface through treatment of Pt/C with a hydrogen peroxide solution, the Pt/C surface can be uniformly coated with an electrolyte polymer; and that since Pt/C is uniformly dispersed within an electrode paste when the paste for electrodes is prepared using the electrolyte polymer-coated Pt/C, a high-performance catalyst layer can be obtained.
Also, JP Patent Publication (Kokai) No. 2006-066138 A does not disclose, an electrode catalyst, but rather a method for producing a separator for fuel cells, comprising forming fine convexoconcaves on the surface of a separator for fuel cells comprising a mixture of graphite particles and binder resin and then hydrophilizing the separator surface using an RF plasma device. This patent document also describes that through the formation of fine convexoconcaves on the separator surface and hydrophilization, the hydrophilicity of the separator and the flooding resistance of the fuel cell are improved.
Moreover, JP Patent Publication (Kokai) No. 63-144153 (1988) A does not describe an electrode catalyst, but rather the fact that carbon fibers can be hydrophilized by carrying out low-temperature plasma treatment for hydrophilization gases containing oxygen, argon, carbon dioxide, ammonia, and nitrogen.